Here's an unusual story looking for resolution real soon:
I recently picked up a case of oil from a local discount-type place. The
stuff in question is Mobil 10-W30, normal automotive stuff. Some might
question the wisdom of using 10-W30 rather than 10-W40, but that is another
story in its own right, and perhaps worthy of discussion here since it might
be (part of) the answer.
So last week I changed the oil in the Spitfire, screwed on a new Wix filter,
poured in the requisit 9+ U.S. pints, and all seemed to be well. I remember
observing at the time that this oil looked really thin, thinner than I ever
remember seeing oil look before. Well, looks can be deceiving (and memory
in retrospect can be selective when you think you have reason to remember
something). So after two days, I'm driving home, sitting at a stoplight, and
the danged oil-pressure light is flickering at me! Hmmm, I never had that
problem before, even with old oil (of un-remembered weight) in the crankcase.
I have an oil pressure gauge, and though it is not banging on zero, it has
been reading low since I changed the oil; even at speed it reads lower than
it used to. I don't know exactly what the pressure is. The sensor is one of
those VDO pickups with both idiot-light and gauge output, designed for a VDO
gauge of range 0-90. My gauge is also VDO but with range 0-160, salvaged
from a VW break-in episode. No matter, the fact is plain: I am not getting
good oil pressure, or at least not as high as I was getting before I changed
the oil, and as low as I have ever seen on this car.
The cure depends on the cause, of course. It is possible that my oil pump is
getting weak (but this engine has only, maybe, 30,000 miles). The oil pump is
fairly easy to replace. Then again, the oil certainly *looks* thin, and the
onset of this symptom was very distinct, correlating to the oil-change. I was
only a minor inconvenience away from changing the oil in the van too, but if
this oil is suspect, I'll pitch it and get something else. Have any of you
ever heard of an episode of bad oil? This makes me very uncomfortable, and
underlines the need to be an informed consumer.
The second consideration is whether this is a natural result of using 10-W30
instead of 10-W40. There are arguments on both sides of the issue. On the
side of using 10-W30 is the "fact" that the higher viscosities are obtained
(supposedly) by additives which break down fairly quickly. I have always held
the opinion that there wasn't all *that* much difference between these two
grades. Certainly I have never seen this big a difference in the apparant
behavior as demonstrated by oil pressure. But perhaps I am mistaken, and with
an aging (?) oil pump, I should not even think of using 10-W30 in the summer.
(Except that it wasn't all that hot yesterday either! Humid yes, hot no!)
A final question (which I also posed to this august group some time ago and
never got much of an answer) is whether an oil pressure overload-valve might
exists on a typical LBC (Spitfire). If so, then there is another issue:
If this pressure relief valve opens at "normal" pressures, then no matter
what oil is run, the pressure downstream is limited. Thus a lower viscosity
will give you higher volume through the channels, which may be beneficial. I
don't know the answer to this one. I'd suspect that if a pressure relief valve
does exist, then it probably doesn't open under most circumstances, so this is
a null issue. Certainly the pump itself is positive-displacement.
In any case, the next step will probably be to junk that oil and get something
else, then change the oil in the Spitfire again. But with what? Do I go with
10-W40 and possibly mask an aging oil pump? I say go with the heavier stuff
and don't worry about the pump. (I don't believe it is bearings yet!) Anybody
wanna' buy some oil cheap?
I await your informed responses. Thanks,
Jim Muller
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